The present invention relates to a semiconductor device having, for example, a redundancy circuit using a fuse to be fused by a laser beam or the like and a method for checking whether the fuse is fused at the time of a shipping test.
In the case of a conventional semiconductor device having a redundancy circuit using a fuse to be fused by a laser beam or the like, it is necessary to check whether the fuse is fused before shipping the device.
In this case, a fuse circuit has a structure in which a pull-up resistance is connected with the fuse in series or a pull-down resistance is connected with the fuse in parallel.
The potential at the connection point between the fuse and the pull-up resistance or between the fuse and the pull-down resistance is determined in accordance with resistance division between the resistance of the fuse and the pull-up resistance or between the resistance of the fuse and the pull-down resistance.
Therefore, because the resistance of a fuse increases when the fuse is fused or it decreases when the fuse is not fused, it is possible to check whether the fuse is fused in accordance with the potential of the connection point of the fuse.
FIG. 1 shows a conventional fuse circuit in which a fuse 1 is connected to a pull-up resistance 2. The first terminal of the fuse 1 is grounded and the second terminal of it is connected to the first terminal of the pull-up resistance 2. The second terminal of the pull-up resistance 2 is connected to a power supply VDD.
The connection point a between the fuse 1 and the pull-up resistance 2 is connected to, for example, an input terminal of an inverter 3. In general, the resistance value of the pull-up resistance 2 is set to a value larger than that of the fuse 1 not fused.
The potential of the connection point a between the fuse 1 and the pull-up resistance 2 is greatly fluctuated depending on whether the fuse 1 is fused. Therefore, it is possible to check whether the fuse 1 is fused in accordance with a signal output from the inverter 3 by using the relation between the potential of the connection point a and the threshold of the inverter 3.
That is, because the resistance value of the fuse 1 is smaller than that of the pull-up resistance 2 when the fuse 1 is not fused, the potential of the connection point a has a value close to the ground. Therefore, the inverter 3 decides that an input signal is "0".
Moreover, because the resistance value of the fuse 1 shows a high impedance when the fuse 1 is fused, the potential of the connection point a has a value close to the power-supply potential. Therefore, the inverter 3 decides that an input signal is "1".
FIG. 2 shows a conventional fuse circuit in which the fuse 1 is connected to a pull-down resistance 5. The first terminal of the pull-down resistance 5 is grounded and the second terminal of it is connected to the first terminal of the fuse 1. The second terminal of the fuse 1 is connected to the power supply VDD.
The connection point b between the fuse 1 and the pull-down resistance 5 is connected to, for example, an input terminal of the inverter 3. In general, the resistance value of the pull-down resistance 5 is set to a value larger than that of the fuse 1 not fused.
In the case of the circuit shown in FIG. 2, the potential of the connection point b between the fuse 1 and the pull-down resistance 5 is greatly fluctuated depending on whether the fuse 1 is fused. It is decided whether the fuse 1 is fused in accordance with a signal output from the inverter 3 by using the relation between the potential of the connection point b and the threshold of the inverter 3.
That is, because the resistance value of the fuse 1 is smaller than that of the pull-down resistance 5 when the fuse 1 is not fused, the potential of the connection point b has a value close to the power-supply potential VDD. Therefore, the inverter 3 decides that an input signal is "1".
Moreover, because the resistance of the fuse 1 shows a high impedance when the fuse 1 is fused, the potential of the connection point b has a value close to the ground. Therefore, the inverter 3 decides that an input signal is "0".
When fusing the fuse 1 by a laser beam or the like, no problem occurs as long as the fuse 1 is completely fused. However, when the fuse 1 is not completely fused, the decision on whether the fuse 1 is fused becomes unstable and a problem occurs.
When the potential determined in accordance with the resistance division between the fuse 1 and the pull-up resistance 2 is kept in a range of a potential at which it is decided that the fuse 1 is fused even if the fuse 1 to be fused is not completely fused and moreover, even if a potential determined in accordance with the resistance division when the fuse 1 is completely fused, a semiconductor device having the fuse 1 is shipped as a non-defective product with the fuse 1 fused.
However, because the potential of the connection point between the fuse 1 and the pull-up resistance 2 or the threshold of the inverter 3 fluctuates due to operating conditions such as voltage, temperature, and noise desired by a user or a change of device characteristics with the passage of time, the above product shipped to the user as a non-defective product may be erroneously decided as a defective product in which the fuse is not fused.
However, if the fuse 1 which must not be fused is erroneously fused due to any reason, the product having the fuse 1 is removed as a defective product because it is decided that the fuse 1 is erroneously fused at the time of a shipping test when the fuse 1 is completely fused.
However, when the fuse 1 is incompletely fused and the potential determined in accordance with the resistance division between the fuse 1 and the pull-up resistance 2 is kept at a boundary between potentials by which it is decided that the fuse 1 is not fused, it is decided that the fuse 1 is not fused and the product having the fuse 1 is shipped.
After shipping the product, however, it may be decided as a product in which the fuse 1 is fused, that is, a defective product when the user uses the product due to the operating conditions such as the voltage, temperature, and noise desired by the user or a change of device characteristics with the passage of time.
Thus, the fact that a product is decided as a defective one after shipping the product to a user results in the fact that the maker loses their credit.
A case is described above in which the pull-up resistance 2 in FIG. 1 is used. However, also when using the pull-down resistance 5 in FIG. 2, the same problem occurs.